1. Field of the Invention
The present invention relates to a method and apparatus for removing embedding media from biological samples on automated instruments prior to immunohistochemical (IHC), in situ hybridization (ISH) or other histochemical or cytochemical manipulations. The present invention also relates to a method and apparatus for conditioning cells or tissues so as to increase the accessibility of various molecules to their respective targets and generally to improve tissue and cell readability.
2. Summary of the Related Art
The diagnosis of disease based on the interpretation of tissue or cell samples taken from a diseased organism has expanded dramatically over the past few years. In addition to traditional histological staining techniques and immunohistochemical assays, in situ techniques such as in situ hybridization and in situ polymerase chain reaction are now used to help diagnose disease states in humans. Thus, there are a variety of techniques that can assess not only cell morphology, but also the presence of specific macromolecules within cells and tissues. Each of these techniques requires that sample cells or tissues undergo preparatory procedures that may include fixing the sample with chemicals such as an aldehyde (such as formaldehyde, glutaraldehyde), formalin substitutes, alcohol (such as ethanol, methanol, isopropanol) or embedding the sample in inert materials such as paraffin, celloidin, agars, polymers, resins, cryogenic media or a variety of plastic embedding media (such as epoxy resins and acrylics). Other sample tissue or cell preparations require physical manipulation such as freezing (frozen tissue section) or aspiration through a fine needle (fine needle aspiration (FNA)). Regardless of the tissue or cell sample or its method of preparation or preservation, the goal of the technologist is to obtain accurate, readable and reproducible results that permit the accurate interpretation of the data. One way to provide accurate, readable and reproducible data is to prepare the tissue or cells in a fashion that optimizes the results of the test regardless of the technique employed. In the case of immunohistochemistry and in situ techniques this means increasing the amount of signal obtained from the specific probe (e.g., antibody, DNA, RNA, etc.). In the case of histochemical staining it may mean increasing the intensity of the stain or increasing staining contrast.
Without preservation, tissue samples rapidly deteriorate such that their use in diagnostics is compromised shortly after removal from their host. In 1893, Ferdinand Blum discovered that formaldehyde could be used to preserve or fix tissue so that it could be used in histochemical procedures. The exact mechanisms by which formaldehyde acts in fixing tissues are not fully established, but they involve cross-linking of reactive sites within the same protein and between different proteins via methylene bridges (Fox et al., J. Histochem. Cytochem. 33: 845-853 (1985)). Recent evidence suggests that calcium ions also play a role (Morgan et al., J. Path. 174: 301-307 (1994)). These links cause changes in the quaternary and tertiary structures of proteins, but the primary and secondary structures appear to be preserved (Mason et al., J. Histochem. Cytochem. 39: 225-229 (1991)). The extent to which the cross-linking reaction occurs depends on conditions such as the concentration of formalin, pH, temperature and length of fixation (Fox et al., J. Histochem. Cytochem. 33: 845-853 (1985)). Some antigens, such as gastrin, somatostatin and α-1-antitrypsin, may be detected after formalin fixation, but for many antigens, such as intermediate filaments and leukocyte markers, immunodetection after formalin treatment is lost or markedly reduced (McNicol & Richmond, Histopathology 32: 97-103 (1998)). Loss of antigen immunoreactivity is most noticeable at antigen epitopes that are discontinuous, i.e. amino acid sequences where the formation of the epitope depends on the confluence of portions of the protein sequence that are not contiguous.
Antigen retrieval refers to the attempt to “undo” the structural changes that treatment of tissue with a cross-linking agent induces in the antigens resident within that tissue. Although there are several theories that attempt to describe the mechanism of antigen retrieval (e.g., loosening or breaking of crosslinkages formed by formalin fixation), it is clear that modification of protein structure by formalin is reversible under conditions such as high-temperature heating. It is also clear that several factors affect antigen retrieval: heating, pH, molarity and metal ions in solution (Shi et al., J. Histochem. Cytochem. 45: 327-343 (1997)).
Microwave heating appears to be the most important factor for retrieval of antigens masked by formalin fixation. Microwave heating (100°±5° C.) generally yields better results in antigen retrieval immunohistochemistry (AR-IHC).
Different heating methods have been described for antigen retrieval in IHC such as autoclaving (Pons et al, Appl. Immunohistochem. 3: 265-267 (1995); Bankfalvi et al., J. Path. 174: 223-228 (1994)); pressure cooking (Miller & Estran, Appl. Immunohistochem. 3: 190-193 (1995); Norton et al., J. Path. 173: 371-379 (1994)); water bath (Kawai et al., Path. Int. 44: 759-764 (1994)); microwaving plus plastic pressure cooking (U.S. Pat. No. 5,244,787;; Pertschuk et al., J. Cell Biochem. 19(suppl.): 134-137 (1994)); and steam heating (Pasha et al., Lab. Invest. 72: 167A (1995); Taylor et al., CAP Today 9: 16-22 (1995)).
Although some antigens yield satisfactory results when microwave heating is performed in distilled water, many antigens require the use of buffers during the heating process. Some antigens have particular pH requirements such that adequate results will only be achieved in a narrow pH range. Presently, most antigen retrieval solutions are used at a pH of approximately 6-8, but there is some indication that slightly more basic solutions may provide marginally superior results (Shi, et al., J. Histochem. Cytochem. 45: 327-343 (1997)).
Although the chemical components of the antigen retrieval solution, including metal ions, may play a role as possible co-factors in the microwave heating procedure, thus far, no single chemical has been identified that is both essential and best for antigen retrieval.
Many solutions and methods are used routinely for staining enhancements. These may include but are not limited to distilled water, EDTA, urea, Tris, glycine, saline and citrate buffer. Solutions containing a variety of detergents (ionic or non-ionic surfactants) may also facilitate staining enhancement under a wide range of temperatures (from ambient to in excess of 100° C.).
In addition to cell surface molecules that may be present on the exterior portion of the cell, other molecules of interest in IHC, ISH and other histochemical and cytochemical manipulations are located within the cell, often on the nuclear envelope. Some of these molecules undergo molecular transformation when exposed to a fixative (coagulative or precipitive) such as form alin. Thus with respect to these molecules it is desirable to not only overcome the effects of fixation but also to increase the permeability of the cell in order to facilitate the interaction of organic and inorganic compounds with the cell.
Other tissue samples may not have been subjected to cross-linking agents prior to testing, but improved results with respect to these tissues is also important. There are a variety of non-formalin methods for preserving and preparing cytological and histological samples. Examples of these methods include, but are not limited to: a) hematology smears, cytospins™, ThinPreps™, touch preps, cell lines, Ficoll separations, etc. are routinely preserved in many ways which include, but are not limited to, air-drying, alcoholic fixation, spray fixatives and storage mediums such as sucrose/glycerin; b) tissues and cells (either fixed or unfixed) may be frozen and subsequently subjected to various stabilizing techniques which include, but are not limited to, preservation, fixation and desiccation; c) tissues and cells may be stabilized in a number of non-cross-linking aldehyde fixatives, non-aldehyde containing fixatives, alcoholic fixatives, oxidizing agents, heavy metal fixatives, organic acids and transport media.
One way to improve testing results is to increase the signal obtained from a given sample. In a general sense, increased signal can be obtained by increasing the accessibility of a given molecule for its target. As in the case for antigens found within the cell, targets within the cell can be made more accessible by increasing the permeability of the cell thereby permitting a greater number of molecules entry into the cell, thereby increasing the probability that the molecule will “find” its target. Such increased permeability is especially important for techniques such as ISH, in situ PCR, IHC, histochemistry and cytochemistry.
Tissues and cells are also embedded in a variety of inert media (paraffin, celloidin, OCT™, agar, plastics or acrylics etc.) to help preserve them for future analysis. Many of these inert materials are hydrophobic and the reagents used for histological and cytological applications are predominantly hydrophilic; therefore, the inert medium may need to be removed from the biological sample prior to testing. For example, paraffin embedded tissues sections are prepared for subsequent testing by removal of the paraffin from the tissue section by passing the slide through various organic solvents such as toluene, xylene, limonene or other suitable solvents. These organic solvents are very volatile causing a variety of problems including requiring special processing (e.g., deparaffinization is performed in ventilated hoods) and requires special waste disposal. The use of these organic solvents increases the cost of analysis and exposure risk associated with each tissue sample tested and has serious negative effects for the environment.
Presently, there is no available technique for removing inert media from sample tissue by directly heating the slide in an automated fashion. Neither is it currently possible to remove inert media from sample tissue while conditioning the sample tissue or cell in a one-step automated staining process.
The methods of the present invention permit a) automated removal of embedding media without the use of organic solvents, thus exposing the cells for staining and thereby reducing time, cost and safety hazards, b) automated cell conditioning without automated removal of embedding media from the sample cell or tissue, c) a multi-step automated process that exposes the cells, performs cell conditioning and increases permeability of the cytological or histological specimens, thereby increasing sample readability and improving interpretation of test data. The methods of the present invention can be used for improving the stainability and readability of most histological and cytological samples used in conjunction with cytological and histological staining techniques.